In recent years, as a form of a technology for improving a data transmission rate, Discrete Multi-Tone (DMT) modulation has been considered actively. DMT is a form of a multicarrier transmission scheme, and data is transmitted by using a plurality of subcarriers. Namely, in the DMT modulation, a plurality of subcarriers are allocated to one communication band. Data is transmitted by using each of the plurality of subcarriers. Therefore, by allocating many subcarriers to one signal band, large-capacity data transmission is realized without increasing a transmission rate of each of the subcarriers.
A communication system including the following transmitter station and receiver station has been proposed. The transmitter station divides a communication band into a plurality of data channels and one or more standby channels, and transmits data by using the data channels and at least one of the standby channels. The receiver station includes a communication quality measurement unit that measures communication quality for each channel for transmitting data, and an adaptive controller that, when there is a data channel with low communication quality, allocates, to the standby channel, at least some bits of bits to be allocated to the data channel with low communication quality. The transmitter station transmits data according to a bit allocation result obtained from the receiver station (for example, Japanese Laid-open Patent Publication No. 2013-48373).
In addition, related technologies are described in Japanese Laid-open Patent Publication No. 2002-26822, Japanese Laid-open Patent Publication No. 2005-244958, and Japanese Laid-open Patent Publication No. 2013-165407.
Meanwhile, as a form of a technology for achieving an increase in a capacity of a communication system, Wavelength Division Multiplexing (WDM) has been widely used. In WDM, data is transmitted by using plural different wavelengths. Namely, in WDM, a plurality of wavelength channels are multiplexed. Thus if DTM modulation is applied to the respective wavelength channels in WDM, it is expected that communication capacity further increases.
However, in WDM transmission, deterioration in quality of an optical signal depends on a wavelength. Therefore, efficient transmission may fail to be achieved only by applying DMT modulation to the respective wavelength channels in WDM.